Such opening devices are used for reducing fibre bales, in particular firmly pressed fibre bales, e.g. of cotton, synthetic fibres and the like, a reduction device being translationally moved over the surface of a bale or a row of bales. This reduction device may consist e.g. of a milling device with pressure rolls disposed axially parallel on both sides of the reduction device, respectively. The milling head is provided with a grate, the grate bars of which are positioned between the milling discs, the opening device and the fibre bales passing each other and the teeth of the milling discs engage into the fibre bales through the grate bars.
Such an opening device, like e.g. the bale opening system Optomix II by Hergeth Hollingsworth GmbH, is provided with a metal detector device. In this case, the grate is suspended insulated at the housing of the milling head, a sudden stop of the apparatus being caused, e.g., upon the occurrence of an electrical contact between a milling disc and a grate bar to prevent damage to the milling device and the occurrence of sparks that could cause a fire.
This metal detector device with an insulated grate allows the detection of metal only when the metal particle, e.g. fragments of the rims around the bales consisting for example of metal bands, present in the fibre bale gets into the region of the working area of a milling disc. With a view to fire protection, this may already be too late, because already the first contact of a metal fragment and the milling disc may lead to sparking.
It is the object of the invention to provide an opening device in which a reliable and early detection of metal is possible without the danger of sparking.
The object is solved according to the invention by the features of the main claim.
According to the invention, the grate bars extend towards the pressure rolls to the immediate vicinity thereof, and the metal detector device includes the grate bars and the pressure rolls. The grate bars form a first contact surface, and the pressure rolls form a second contact surface, and define a gap therebetween whereby the pressure rolls and the grate bar serve as a metal detector.
In a preferred embodiment it is provided that the pressure rolls consist of a roll with a metallic surface, insulated against the casing. In this way, the grate does not have to be insulated against the casing, which eliminates complex holding means otherwise necessary with a view to the great pressure forces occurring when opening a bale.
In a double-roll opening system with mutually offset milling discs arranged on two parallel rolls, the grate bars immediately extend from the one pressure roll to the working area of the milling disc arranged in front of the other pressure roll. In this manner, the grate bars overlap in the central portion between the milling rolls, the bars being mechanically and electrically connected via a fixing means extending parallel to the milling rolls.
It is provided in a further embodiment that the pressure rolls and the milling discs form the ground potential of the metal detector device together with the casing, whereas the grate is insulated and mounted at the casing. In this way, it is possible to detect metal rather early, i.e. both at the level of the pressure roll and at the level of the milling disc, since the teeth of the milling rolls also have the ground potential.
The gap between the ends of the grate bars and the working or engaging range of the pressure rolls or the opener discs, respectively, has a width of approximately 5 to 15 mm, preferably 8 mm.